Django 2.2-LTS support

- New development settings and logging
- PostgreSQL is the default database for development
- Default language is set to en-us, timezone is set to UTC
- Upgrade python packages
- Fix Travis CI Test
- Fix and improve SoftDelete operations
- Improved tests
- Change pylintrc: Now 119 chars per line
- Add pre_undelete and post_undelete signal dispatcher
- Fix #17 : Typos in Rakefile
- Fix #7  : OverwriteStorage integration
- Fix #6  : ImageField now uses AdminImageFileWidget
- `urlify` now uses external and better library